The present invention relates to an image display device.
Image display devices are generally categorized into two typesxe2x80x94non-emissive type and emissive type. Non-emissive type devices have an external light source means and employ a display element which modulates the light from the light source means in order to display images. Examples include liquid crystal monitors, liquid crystal projectors, and the like. Emissive type devices do not have an external light source means but the display element itself emits light to display images. For example, CRTs, PDPs, FEDs, organic ELs, and the like fall under this type.
For these conventional image display devices, high luminance, high contrast, high resolution, and lower power consumption have been desired to improve the picture quality. Of these, luminance has the greatest influence on the viewer""s perception of images and therefore is the most important parameter.
In view of this, various attempts have been made in the past to make luminance uniform over the display screen. Such attempts include, for example, in the case of non-emissive type devices, using the characteristics of the light source means, and in the case of the emissive type devices, varying video signal in an appropriate manner, in order to achieve more uniform display screen luminance.
Taking a liquid crystal display device as an example, a conventional technique for making the luminance uniform within the screen is discussed below. The liquid crystal display device comprises, as shown in FIG. 32, a liquid crystal display element 1901, a backlight 1902, and a driving means 1903 for the liquid crystal display element 1901. The backlight 1902 comprises at least a light source 1904, a transparent light guiding plate 1905 for supplying light from the light source to the liquid crystal display element 1901, and a reflective cover 1906 for covering the light source. ON the back surface of the light guiding plate, a plurality of scattering microdots 1907 are formed numerously so that the luminance within the plane is controlled by the shapes and positions of the formed scattering microdots 1907.
The light discharged from the light source 1904 enters from an end face of the light guiding plate 1905 and is transmitted inside the light guiding plate, repeating the total reflection. All or part of the light which has entered the scattering microdots 1907 changes traveling direction, and the light which is incident on the upper surface of the light guiding plate at an angel smaller than a critical angle is discharged as output light, entering the liquid crystal display element 1901.
Accordingly, the distribution of scattering microdots on the back surface determines the luminance distribution in the screen; the conventional backlight 1902 has such a configuration that the areas of the dots are made greater from the peripheral portion of the screen towards the center portion and thereby the distribution of screen luminance is made 80% or greater such that uniform brightness is achieved.
For example, let us assume a case where light sources are horizontally disposed at end faces of the light guiding plate, the end faces being at the top and bottom of the screen. It has been known that, supposing the areas of the scattering microdots on the back surface are uniform over the entire screen, the luminance distribution is mostly formed in a vertical direction (along the y axis), resulting in a distribution in which a region including the center is dark, as shown in FIG. 33. This is due to the fact that a large portion of the light is scattered in portions of the light guiding plate which are near the light source and is discharged therefrom.
In order to compensate such a luminance distribution, it has been suggested that the areas of the scattering microdots be varied such as to be proportional to the reciprocal of the luminance distribution obtained in the case when the areas of the scattering microdots are uniform over the entire area of the display screen. That is, the areas of the scattering microdots are varied in the vertical direction in the screen so that the scattering microdots nearer to the center have larger areas. Thereby, uniformity of the luminance within the screen can be increased to as high as 80% or higher.
Next, a case of an emissive type display is described as an example. In a emissive type, it has been suggested that non-uniformity in the display element itself be corrected. Specifically, in order to compensate the luminance variation between the pixels, luminance variation is compensated in each pixel by only the varied value.
Generally, a driving means of a display device comprises a video signal decoding means 2101, a signal correcting means 2102, and a display element interface means 2103. The video signal decoding means 2102 serves the purpose of producing RGB color signals and horizontal and vertical synchronizing signals from ordinary NTSC signals.
The signal correcting means 2102 corrects each signal of R, G, and B and, essentially, corrects gray level characteristics in consideration of input-output characteristics of a display element 2104. The display element interface means 2103 serves the purpose of adjusting the corrected signal to match a signal level of the display element.
The signal correcting means 2102 is provided primarily for the purpose of achieving good gray level characteristics; however, when the image display means 2104 has some factors leading to luminance variation, it additionally comprises a means for correcting luminance variation. For example, there are cases where luminance variation between pixels, for example, is caused by inaccuracy in the production of display elements. In such cases, to make luminance in the screen uniform, gray level characteristics is varied for each pixel so that the luminance is made uniform at a certain level. More specifically, the signal correcting means comprises, in the form of an integrated memory, a lookup table which defines a gray level characteristic for each of the pixels, and a table lookup is performed synchronized with the synchronizing signals, so that luminance is appropriately modulated.
As described above, in prior art display devices, various attempts have been made to make brightness uniform in the display screen.
In recent years, display screen sizes have been increased, and even for home use TVs, 20-inch or larger screens have been desired. However, conventional display devices have drawbacks in that power consumption considerably increases as screen sizes increase. When a given luminance is required, the amount of power consumed increases in proportion to the area. Moreover, when a higher resolution is required as display size increases, the area per each pixel becomes smaller and therefore efficiency generally reduces. For this reason, when larger screen sizes and higher resolutions are desired without changing the luminance, the amount of power consumed increases even further.
Thus, if the luminance in the display screen is kept uniform, an increase in power consumption is inevitable. In addition, simply reducing luminance may make it possible to suppress an increase in power consumption, but the image will appear dark to the viewer of the image.
It is therefore an object of a first aspect of the present invention to provide a display device which is capable of reducing power consumption while displaying images that can create the viewer impression of bright images. More specifically, in order to solve the foregoing and other problems, there is provided in accordance with the first aspect of the present invention a display device in which the luminance is gradually decreased from the center of the display screen towards the peripheral portion, and by making the luminance gradient less perceivable utilizing a certain type of optical illusion, a reduction in power consumption is achieved without impairing viewer perception of a bright image.
It is an object of a second aspect of the present invention to make brightness distribution in the display screen less perceivable by utilizing a certain type of psychological illusion, even when the luminance in the peripheral portion is decreased.
In order to solve the foregoing and other problems, the present invention provides, according to the first aspect of the invention, a display device comprising at least an image display means and a luminance gradient forming means for forming a luminance gradient, wherein the luminance gradient forming means is such that, when displaying a full-white signal, a luminance of the image display means substantially monotonously decreases from substantially the center of a display screen of the image display means towards a peripheral portion thereof. The term xe2x80x9cmonotonouslyxe2x80x9d is understood to mean xe2x80x9ccontinuously.xe2x80x9d
Specifically, a display device according to the first aspect of the invention does not have substantially a uniform luminance over the entire display screen but is imparted with a luminance gradient which substantially monotonously decreases from the center of the screen toward the peripheral portion, and thereby, the power consumption is reduced in comparison with cases where the luminance is made substantially uniform over the entire screen.
In this configuration, it is preferable that, in order to make the luminance gradient less perceivable for a viewer, the luminance of a display image be substantially monotonously decreased. When this is the case, it is more preferable that the luminance is monotonously decreased from the center of the display screen in a horizontal or vertical direction since luminance gradients having symmetry are even less noticeable. For similar reasons, it is preferable that the luminance gradient be substantially symmetrical with respect to a vertical axis through substantially the center of the display screen or a horizontal axis through substantially the center of the display screen.
From these viewpoints, it is considered that there are several preferable examples of distribution profile of luminance gradient. For example, when the luminance gradient is distributed so as to be a concentric circle-like profile, the luminance gradient is very difficult to perceive. The term xe2x80x9ca concentric circle-like luminance distributionxe2x80x9d is intended to mean a distribution such that the lines connecting the points having approximately the same luminance form substantially circular shapes centered on substantially the center of the display screen. Luminance distribution profiles are defined in a like manner throughout herein.
The above-described configurations makes the luminance distribution less noticeable for the following reasons. Since human pupils are circular, the region that a human is capable of perceiving at one time has a near circle shape. Consequently, when a luminance gradient distribution profile in the display screen is circular, the region that a human is capable of perceiving at one time and the distribution profile of the luminance gradient are approximately similar in shape, and for this reason, psychological effect of the luminance gradient becomes small due to a kind of optical illusion. The luminance distribution profiles need not be substantially circle-like profiles and similar effects are obtained with ellipse-like profiles and rhombus-like profiles, so it is also preferable to form these luminance distribution profiles.
In the cases of ellipse-like luminance gradients, the ratio of the major axis and the minor axis may be made substantially equal to the ratio of the horizontal length and the vertical length of the image display screen to make the luminance gradients less noticeable. As a consequence, the shape of appearance of the display image and the distribution profile of the luminance gradient are nearly similar in shape, and therefore, psychological effect of the luminance gradient becomes small due to a kind of optical illusion. For similar reasons, a rectangle-like luminance distribution is also one of the examples of preferable luminance distributions.
Such luminance gradient may be defined by mathematical functions. Now, let us consider a luminance gradient function f(x,y) which is substantially equal to a luminance B at a point in the display screen which has a distance x from the origin in the horizontal direction and a distance y in the vertical direction, the origin being substantially the center of the display screen. Here, when the luminance gradient function f(x,y) is expressed with a luminance distribution profile function r(x,y) as fx,y)=f(r), luminance gradient can be expressed in a simpler manner.
Since luminance gradient is less noticeable as the slope of luminance gradient becomes more gentle, it is preferable that the luminance gradient function f(r) be monotonously decreased with respect to the variable r of the luminance distribution profile function. One example is a case of a linearly decreasing luminance gradient. Another example is that of a luminance gradient that decreases in an exponential function-like manner with respect to r.
This configuration is even more preferable because the slope of the luminance gradient is small in the vicinity of the center of the display screen but becomes larger towards the outward portion, whereby luminance gradient is even less perceivable, compared to the case where a luminance linearly decreases. This is due to the fact that when a human observes an image, he or she usually has a tendency to gaze mainly at the center of the display screen. That is, the luminance gradient in the portion at which a viewer focuses is small while the luminance gradient in the peripheral portion of the display screen, which is not intensely viewed, is large and therefore, the luminance gradient in the display screen is made less perceivable. For similar reasons, a luminance gradient in which the luminance decreases according to powers of r and a luminance gradient that sinusoidally and monotonously decreases towards the peripheral portion are also preferable.
It is preferable that the degree of luminance gradient image viewers can tolerate (how low the luminance in the peripheral portion can be made) be determined so as to conform to the results of human-engineering-based subjective assessments. In other words, it is preferable that a luminance gradient in a display screen match with a variety of threshold values defined according to the results of subjective assessments. These results are generally referred to as detection limit, permissible limit, and tolerable limit (limit for practical use).
According to the results of the subjective assessment experiments carried out by the present inventors, the threshold luminance values which are defined by the ratio of the luminance at the boundary of the display screen region having a screen diagonal ratio of 0.9 to the luminance of the center portion when a full-white signal is displayed are 55% xc2x115% for the detection limit, 30%xc2x110% for the permissible limit, and 15%xc2x15% for the tolerable limit (limit for practical use). Thus, it is preferable that luminance gradients be determined so as to satisfy the above conditions.
When a luminance gradient satisfies the condition of the detection limit, (the luminance at the boundary of the display screen region having a screen diagonal ratio of 0.9 is about 55% of that of the center of the display screen), 50% of the viewers do not perceive the luminance gradient. When a luminance gradient satisfies the condition of the permissible limit, the luminance gradient is permissible for 50% of the viewers and power consumption is further reduced compared to the case where the condition of the detection limit is satisfied. Likewise, when the condition of the tolerable limit is satisfied, power consumption is reduced even further.
This technique of reducing power consumption by forming a luminance gradient in the display screen using a luminance gradient forming means can be applied to any display devices that comprise a non-emissive or emissive type image display means. In the cases of display devices having a non-emissive image display means, it is possible that a light source means that provides light to the non-emissive image display means be provided with a luminance gradient forming means. For example, a display device using a transmissive liquid crystal panel as the non-emissive image display means has a light source and a light guiding plate as the light source means. By using a distribution of scattering dots formed on the back surface of the light guiding plate, a desired luminance distribution can be formed.
Either of non-emissive display means and emissive display means is capable of forming a desired luminance gradient by modulating input signals with a luminance gradient forming means. In this configuration, the luminance gradient forming means may comprise a lookup table that determines a gray level characteristic for each pixel, whereby the luminance distribution in the display screen is formed into a desired profile.
Likewise, it is possible to employ a configuration in which the luminance gradient forming means for varying the gains of the level shifter may be provided in the interface portion with the display element.
When the image display means is an FED, a desired luminance gradient can be formed in the display screen by providing an extraction voltage varying means as the luminance gradient forming means.
A display device according to a second aspect of the present invention comprises at least a luminance gradient forming means and an image display means, wherein brightness index defined by Eq. (1) substantially monotonously decreases from substantially the center of the display screen towards the peripheral portion thereof.
Eq. (1) has been established from the subjective assessment experiments carried out by the present inventors and is a criterion that shows a good correlation with psychological impression concerning brightness.
As discussed in the section describing the first aspect of the invention, that a luminance monotonously decreases from the center of the display screen towards the peripheral portion means that a brightness index also monotonously decreases. This configuration is, as discussed in the section describing the first aspect of the invention, effective for reducing power consumption. However, because the brightness index simultaneously decreases, psychological impression concerning brightness degrades according to the equations. In view of this, in the first aspect of the invention, the profile of luminance gradient is controlled or the slope of luminance gradient is monotonously decreased in order to make luminance gradient less perceivable as luminance unevenness.
On the other hand, in the second aspect of the invention, brightness index that decrease according to the equations are improved in order to improve viewer""s impression on brightness. Specifically, it is an object of the second aspect of the invention that degradation in the viewer""s impression of brightness due to decrease in luminance is improved by optimizing gray level characteristics, whereby luminance gradient is made even less perceivable. This configuration merely varies gray level characteristics, so basically, power consumption does not increase.
In this configuration, brightness index may be compensated so as to be substantially uniform over the entire display screen area, whereby the luminance in the peripheral portion is reduced without degrading the viewer""s impression of brightness.
As well as compensating brightness index so as to be substantially uniform over the entire display screen area, it is also possible that brightness index may be compensated within the region having a screen diagonal ratio of 0.5, for example. The reason is that when a human observes an image, the point at which he or she focuses on most is in the vicinity of the center portion of the display screen and the area that he or she can observe at one time is substantially within the region having a screen diagonal ratio of 0.5. Thus, by optimizing gray level characteristics in the display screen and thereby improving the distribution of brightness index, power consumption can be reduced without adversely affecting viewer""s impression of brightness.
It is also preferable that the distribution profile of brightness index in the display screen be corrected. When the preferable luminance profile as described above is not attained due to individual differences of display elements, for example, it is possible to make luminance gradient less perceivable by making brightness index have a desirable distribution profile using a brightness index improving means, that is to say, by forming a distribution profile having high degree of symmetry as described above.
In a similar manner, even when the actual luminance slope is shifted from a desired luminance slope, it is possible to make luminance gradient even less perceivable by matching brightness index with the desired luminance slope.
Additionally, it is possible to make less perceivable luminance unevenness in the display screen which is often a problem in a liquid crystal display element having a direct type backlight. As for a direct type backlight, which has a configuration such that several light sources are provided directly beneath a liquid crystal display element, the portions directly above the lamps are bright while the portions between the lamps are dark.
It is possible, however, to ameliorate picture quality degradation due to such luminance distribution, because the brightness index improving means according to the second aspect of the invention is capable of making brightness index uniform in the display screen by appropriately adjusting gray level characteristics so that reduction in luminance is compensated.